The process of signal transduction is an essential function in all organisms, enabling cells to sense and respond to constantly changing environmental conditions and, in multicellular organisms, to successfully communicate with other cells. One of the prominent and pivotal cellular responses is the alteration of specific gene expression. Comprehension of the signal transduction pathways that lead to regulation of gene expression is fundamental for understanding the dynamic nature of cells. Investigation of the mechanisms by which gene expression in plants is regulated in response to a variety of environmental stimuli is immediately possible. The expression levels of five genes, the TCH genes of Arabidopsis, are rapidly and sharply increased in response to diverse stimuli, such as rain, wind, touch, heat shock and darkness. Three of the TCH genes encode calmodulin-related proteins, suggesting roles for calcium ion and calmodulin in the signal transduction pathways. The TCH cDNAs are powerful molecular tools to analyze and compare how seemingly unrelated stimuli control a common cellular response. Determination of the cis-acting sequences that confer regulation in cultured cells and in intact plants and the mechanism(s) of TCH gene induction will reveal the regulatory machinery that functions to respond to different stimuli. A long-term genetic dissection of the mechanosensory pathway will be initiated by screening for mutants that fail to sense and/or respond to simple mechanical stimulation. These mutants will be critical for the identification of other steps and components that function in the mechanosensory pathway of Arabidopsis. Because of the universal role of calcium ion as a second messenger in eukaryotic cells, elucidation of how calcium ion may regulate expression of genes, including genes encoding calmodulin and calmodulin-related proteins, will likely be relevant not only for plant cells but also for the great variety of animal cells that translate extracellular stimuli, such as hormones and electrical impulses, into calcium signals.